Rigid urethane foam is an industrial material having excellent heat-insulating property, moldablility and field processability and, as such, has been used in a broad spectrum of applications such as the heat insulation of electric refrigerators, buildings, cold storage warehouses, storage tanks, refrigerator ships and pipeline systems.
A typical rigid urethane foam is manufactured by mixing and reacting a first component A the principal ingredients of which are a polyol, a catalyst, a foam stabilizer and a blowing agent and a second component B the principal ingredient of which is a polyisocyanate.
The blowing agent generally used in the manufacture of urethane foams is trichloromonofluoromethane (hereinafter referred to as CFC-11).
However, chlorofluorocarbon (hereinafter referred to as CFC), of which CFC-11 is a species, is chemically so stable that it diffuses into the stratosphere where it destroys the ozone layer. As a result, the ultraviolet rays from the sun are not absorbed into the ozonosphere but reach the surface of the earth to increase the risk of skin cancer, for instance, thus causing serious environmental problems in recent years. Therefore, regulatory control is coming into effect in 1989 over the use of CFC. CFC-11, used in the manufacture of urethane foam, is also subject to the same control.
Under the circumstances, many explorations are in progress for finding blowing agents which may take the place of CFC but up to this day no other useful blowing agent has been developed as yet. HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane), HCFC-141b (1,1-dichloro-1-fluoroethane) and so on have been proposed as substitutes for CFC-11 but none have been implemented on a commercial scale and, hence, we have had no industrially effective method of reducing the CFC requirements.
Barring its adverse effects on the environment, CFC-11 is a very useful material offering the advantages of (1) adequate boiling point, (2) low thermal conductivity in gaseous state, (3) adequate solubility and, hence, viscosity-reducing property, and (4) noncombustibility and low toxicity.
It is a great headache to the industry that it is now obliged to spare the use of this useful substance, CFC, and in the absence of a substitute blowing agent on the market, the only alternative means available is the utilization of water as a blowing agent. While the quantity of water used generally ranges from about 0.2 to 3.0 weight parts as pointed out in Japanese Patent Publication 53-9797/1978, the use of water in greater amounts would cause the following troubles.
(1) Compared with the urethane foam rich in CFC-11, the proportion of CO.sub.2 in the gaseous phase of the product urethane foam is larger, with the consequent deterioration of heat-insulating property. PA1 (2) Generally, the product urethane foam becomes friable. PA1 (3) As the proportion of CFC-11 is decreased, the viscosity of the system is increased so that the system becomes incompatible with the foaming machine. PA1 (4) The adhesive property of the urethane foam for the mating surface, particularly the initial adhesive strength, is seriously affected. This decrease in initial adhesive strength increases the incidence of detachment of the foam from the refrigerator or other insulation panel surface. This is a fatal drawback for any urethane foam, of which self-adhesion ought to be one of its basic requirements. PA1 (5) Because of the vigorous exothermic reaction between water and a polyisocyanate, foaming occurs with increased intensity so that the cell walls are destroyed by internal gas pressure to create local open cells. This would result in enhanced thermal conductivity and/or increased water absorption.